The present invention relates to a method to enhance the recovery of landfill gases including methane from landfills by recovery wells.
Landfill recovery wells are used in landfills to remove methane and other landfill gases, (methane is the predominant gas of the landfill), that are naturally produced in landfills. This removal prevents methane from building up to potentially dangerous levels. In addition to the safety concern, the methane produced can be of significant quantity for useful energy use such as generating electricity.
Methane is a by-product of specialized bacterial metabolism known as methanogenesis. Methane (CH4) is a product of methanogenesis and results from degradation of organic compounds in a strictly anaerobic environment. These organic compounds can include many different items including wood, paper, carpets, sludge, or any organic waste. Methanogens are strictly anaerobic and die instantly in the slightest presence of oxygen. The methanogens that are strict aerobes can also exist in strict anaerobic micro-environments created by other aerobic microorganisms. Methanogenesis is a natural process that will take place in the presence of organic compounds and a strict anaerobic environment.
The landfill gas extraction wells can be completed in a variety of material including compacted to loose material, and can vary significantly in landfill gas recovery. The amount of landfill extracted is largely dependent upon the degree of compaction, and the nature of the material that the well is completed in. Most of these wells are constructed of PVC, High-Density polyethylene (HDPE) or similar material. The largest portion of these HDPE wells are slotted thus allowing the methane to travel through the fissures and pores and into the landfill recovery well. The largest portion of the total depth into the landfill is perforated. Most of the perforated section of these wells is unsaturated and only a small length in the bottom of the well may contain some type of leachate. As will be discussed later the unsaturated nature of these wells makes some of the traditional procedures used for well rehabilitation limited. Over time these fissures and pores become plugged with a variety of deposited material and more significantly compaction. This deposited material may consist of many different things including mineral deposits precipitated from the condensate or leachate, biological slime and byproducts of reactions between landfill materials. In addition to the deposited material the landfill continues to compact thus blocking off many of the fissures and pores that existed when the wells were initially installed. The compaction is possibly the more significant aspect in reducing methane recovery than deposition of material on the surfaces of the well screen and surrounding landfill material. Compaction is a significant problem on many landfill sites where subsidence creates sink holes, breaks in the landfill covers and shearing of the wells causing breakage.
Current well rehabilitation procedures are not well suited to work in unsaturated or semi-saturated environments. Many advances have been made during the last decade in understanding well problems and well rehabilitation solutions. Traditional well rehabilitation procedures rely on a fluid in the well (most often water) to transport the energy to the zone of the well where it is required to dislodge, disrupt and remove the plugging material. This energy is often in the form of chemical or mechanical energy.
There have been recent attempts at procedures trying to overcome these limitations. In addition to not being able to transport energy into the landfill, that surrounds the landfill gas recovery wells, the current rehabilitation procedures are limited in their ability to fracture and create fissures. The traditional rehabilitation procedures used have demonstrated limited results in enhancing landfill gas recovery from these wells. It is an object of this invention to overcome these prior art problems.
The use of gaseous and liquid carbon dioxide applied during the present rehabilitation process has been demonstrated to significantly enhance landfill gas (predominately methane recovery). The energy contained in gaseous and liquid carbon dioxide has the ability to penetrate the landfill significantly further than other procedures. This energy can be carried into the landfill with or without leachate or fluid in the well. The liquid carbon dioxide can create fissures in the landfill and allow the methane to enter the well. The snow (a phase of carbon dioxide produced when the pressure of liquid carbon dioxide drops below 70 PSI) produced can also create fractures and release energy over a longer period of time. In addition to the energy being delivered into the landfill without saturation, it is also possible to add fluid into the well during the injection process in order to achieve more penetration.
Another aspect of the present procedure utilizes rapid pressurization and depressurization repeated in cycles. This cyclic event can be repeated until flow of liquid and gaseous carbon dioxide flows more freely (a lower pressure in the well) into the landfill surrounding the well screen. Each time the flow is increased into the landfill the fractures and fissures can be created further away from the well. With an increasing network of fissures surrounding the well, landfill gases including methane can more easily be transported towards and into the well.
Carbon dioxide, in addition to having tremendous energy also reduces surface tension allowing it to penetrate into the surrounding landfill. The energy contained in gaseous, liquid or solid carbon dioxide can also effectively remove any of the biological or mineral deposits that may also impede the transportation of methane through the fissures and openings in the fill or well screen.